Wood as a Fuel No. 8 UI Extension Forestry Information Series

Wood-Burning Furnaces Roy Taylor

If you’re thinking of heating your entire home with a Furnaces require periodic stoking, as often as every wood stove, you’ll probably be disappointed. A 10 hours. A wood furnace connected to the ductwork wood-burning furnace rather than a stove is the wiser of a conventional furnace can be used to heat the purchase. entire house. Thermostatic controls can activate the conventional furnace when the wood fire dies out Wood stoves are good for heating small houses or one (Figure 1). Some furnaces have dual heating chambers or two rooms, but most homes are designed for with the wood-burning side connected thermostatically central heating with a furnace. Some wood furnaces with the conventional side (Figure 2). are made to fit a hot air duct system, others a hot water system. Two methods for easing the care required of wood- burning furnaces are the self-feeding wood hopper and Knowing something about what it takes to operate a the wood-burner/thermal-storage units. Self-feeding wood furnace may help you decide whether or not to wood hoppers hold larger wood supplies than ordi- invest in one. You’ll have to work harder to keep the nary furnaces. As the long pieces of wood burn off on wood furnace going than you would with a gas or oil the lower ends, gravity causes them to slide down so burning furnace. If you’re cutting your own wood, the that new fuel gets into the combustion zone. Figure 3 splitting, stacking, and hauling can be tiring and time shows an inclined hopper capable of holding several consuming. Also, while wood can be purchased from large chunks of wood up to 4 feet long. dealers already cut and seasoned, buying large amounts of prepared wood can be expensive.

Figure 1. Wood-burning furnace attached to conven- Figure 2. Cutaway of two-fuel furnace. tional furnace/air-duct system. flues cold air hot air duct from hot air duct to house plenum house dividing partition hot air duct between air oil heating plenum burning and combustion flue to fire box air supply chimney existing chamber furnace air supply wood-burning for firebox cumbustion wood burning furnace cold air return draft control Figure 3. Log burning furnace. Furnaces can be thermostatically controlled. When heat is demanded by the thermostat, the air supplies hot air ducts to house flue to are fully opened so that the fire will burn at a high rate. chimney The furnace is equipped with a blower that circulates secondary air the heat throughout the home. loading hatch A wood furnace requires more maintenance than a gas primary or oil burning furnace. You must remove and clean the air chimney periodically to cut down on the accumulation of soot and creosote.

This information first appeared as CIS 482 and was part of the combustion Wood as a Fuel Series. chamber About the Author: Roy Taylor is a former Extension Educator firebrick - Agricultural Engineering and Professor at the University of grate Idaho.

ash pit Figure 5. Wood-burner/thermal storage unit.

Figure 4 shows a self-feeding wood furnace designed at the University of Maine. Characterized by improved air and temperature controls, this furnace achieves particularly high burning efficiencies. Wood-burner/thermal-storage units are designed to burn wood at efficient high temperatures for short large periods of time. Heat is stored in insulated water tanks thermal or rock storage bins and used as needed to heat the mass house. An advantage of this system is that the furnace connections flue pipes does not need to run continuously, perhaps only every to house heat insulation 2 or 3 days. The burner and storage unit are usually distribution located outside of the house with pipes or ducts system fire box heat exchanger connected to the house’s heat distribution system (Figure 5).

Figure 4. High efficiency stick-burning furnace. chimney

heat exchanger secondary air connected to house heating system refractory tunnel

primary air

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